Most games have their own structure specific to the game but how Windows vs Linux versions (or Windows vs Windows via WINE) store / read the save are often the same. I've had many ScummVM, Stardew Valley, GZDoom, etc, game saves made in Windows that loaded in Linux work fine. Even things like different Windows (CR LF) vs Linux (LF) vs Mac (CR) carriage return markers in text based save files are often convertible via Notepad++ (Edit -> EOL Conversion). Same with ANSI vs Unicode (Encoding Menu). So even if a game is finicky about how a text based save file is read under a different OS, that can often be fixed in a few seconds.

How common it is isn't relevant, though. It was one example among many. You will have a hard time finding any data format that has incompatibilities between different operating systems.

The only one I can think of offhand, as already mentioned, are text files, where Windows uses \r\n for line breaks whereas macOS and Linux use \n. However, that's convention and not something strictly enforced by the OS. If your game uses text files for saves and you program it to specifically use only \n for line breaks, it will work fine on Windows. Just be aware that users might load the files into Windows text editors which will usually convert \n to \r\n when re-saving. Generally they all have their own structure. Occasionally it's something like XML but usually some custom binary stuff. As I said, data is data. There's nothing magic about writing a file with a particular operating system that makes an arbitrary series of bytes somehow not compatible with another operating system. But the question is why, not why not. The differences when porting a game are things like graphics/sound API code, not any of the game logic, which includes save files. Introducing OS-specific changes in game logic for no reason sounds like a fantastic way of creating bugs and unnecessary problems.

In case it wasn't clear, I'm not speaking hypothetically about this; I've occasionally shuffled save files between different OS versions. To be fair, your concern would be more valid in the days of porting between different CPU types where e.g. PPC was big-endian and X86 was little endian; that could and did cause headaches, since data could be fundamentally stored and used differently. However basically everything is little-endian these days, including Arm chips typically used in phones and Apple's new M-series chips.

The request is still valid. Perhaps one wants to cease using the client (and perhaps on a computer that didn't have it previously). This would ALSO work well to combine with the request that we be able to see which games do have cloud saves and manage them (e.g., delete) -- which is not available at all, client or otherwise.

This.
Like ya if you used the client and want to stop using client and cloud saves then it is just the persons obligation to get/keep the offline saves in order.

This isn't doing cloud saves. This is downloading your archives/backups of your data that GOG already has.

The client (which is essentially just a web browser to GOG's sites and APIs already) already has this functionality, zipped up on the server and downloaded as a file. GOG just needs to add it to the game page on the site instead of hiding the link.

And, as I said, it's already a necessary request to be able to manage your cloud saves, e.g, delete/clear them. Or even expose for which games you have cloud saves. It all should be done to be proper.

In this case he switches to a operating system that does no longer support the client and wants his old data.

The scenario is similar to a google/facebook/whatever account.
You can close the account, then request that they send you the data they collected about you.

It would be easy for GOG to provide a download link that contains all the save files for his user profile or make the API that Galaxy uses to download the savefiles publicly available (still requiring a login of course). In the latter case some programmers could come up with solutions like they did for the offline installers.

Well I don't use cloud saves. I think they are dumb. It's ironic when so many people clamber for completely DRM free gaming but also want want their data saved online. If you can move your game to an OS that does not support your game I think you can backup your own save.

But either way you are wrong. You cannot adequately control the state of saves under all conditions unless you have a program running locally. Doing it via web browser would not work.

Even if you cleared them on the server, they'd still be on your machine. And then when the game exited the client would save them on the server again. So it would be confusing to people using a web browser and thinking the saves were clear when they can't be. Only a local client can really clear them.

And for the people who know how to clear them locally, they also know how to back them up locally. They don't need cloud saves.

Clearly you have no freaking clue how anything words, including GOG Galaxy's EXISTING cloud save backup system... or why one might need to clear their cloud saves when they cause problems (for people who DO want to use the client too!), please excuse yourself from this thread with your uninformed and useless opinions.

The old data should be already mirrored locally anyway or well downlodaed before switching the operating system.
Expecting just because you wiping your old os and having not backupped your saves and then deciding to opt in for a system not supporting galaxy is just close to user error

I have been doing cloud saves for years without any kind of client.
Obviously, I am not using GOG servers to store my saves.

Your post makes one question the purpose of cloud saves in general because most scenarios where one has use for them are based on a user error.

Who mirrors his saves locally, does not need cloud saves.

You're using software other than the game, AKA a client.
I know how saves work. I've written plenty of code to do so. But it's all irrelevant because the situation is what the situation is. Nothing's changing.

nvm

Man, this community sometimes...

"Hey guys, I would like my cloudsaves back"

People respond:

"I don't use cloud saves. I think they're dumb."

Then go onto arguing about cloud saves' existence and purpose.

Brilliant! I think we all have a career in tech support ;)